This invention relates to a method of determining activation of an exhaust gas concentration sensor, and more particularly to a method of determining activation of an exhaust gas concentration sensor having an exhaust gas concentration-detecting element and a heater for heating the element.
In general, an oxYgen concentration sensor (hereinafter referred to as "the 0.sub.2 sensor") is used as an exhaust gas concentration sensor for use in the air-fuel ratio feedback control of an internal combustion engine. The 0.sub.2 sensor cannot accurately detect the concentration of oxygen unless it is used in a sufficiently activated state. Therefore, it is necessary to determine whether or no activation of the 0.sub.2 sensor has been completed, and it is well known to determine that the 0.sub.2 sensor has been activated when the output voltage of the 0.sub.2 sensor becomes lower than a predetermined reference value (e.g. 0.4 V).
However, according to this method, in some cases, e.g. in a case where the engine is restarted after completion of warming-up of the engine, the 0.sub.2 sensor is erroneously determined to be inactive although it has actually been activated. Therefore, there has been proposed, by U.S. Pat. No. 4,759,332, another method of determining activation of the 0.sub.2 sensor in which the predetermined reference value is set to a higher value before a predetermined time period elapses than after the predetermined time period has elapsed.
In the meanwhile, it is conventionally known to provide a heater for the 0.sub.2 sensor in order to reduce the time required for activation of the 0.sub.2 sensor when the weather is cold or on like occasions.
In the case of the 0.sub.2 sensor equipped with the heater, activation thereof is usually completed if the heater has been energized over a predetermined time period while it is operating normallY, irrespective of the warmed-up condition of the engine, the ambient temperature, etc. However, there is a case where the air-fuel ratio is enriched due to fuel attached to inner wall surfaces of the intake pipe, so that the output voltage of the 0.sub.2 sensor continues to assume a high value (e.g. approximately 1.0 V) even when the sensor has actually been activated. In such a case, the 0.sub.2 sensor is determined to be inactive, and hence the start of the air-fuel ratio feedback control is delayed, which adversely affects purification of exhaust gases.